FIGS. 3A–3C depict the frame, normal reverse link slot format, and abbreviated reverse link slot format, respectively, for an exemplary prior art digital Time Division Multiple Access (TDMA) cellular air interface known in the art as IS-136 (see, for example, Section 4.4 of IS-136.1, Rev. A, Mar. 21, 1996 and IS-136.2, Rev. A, Feb. 12, 1996). The reverse link is considered to be in the direction from a mobile station (MS) to a base station (BS), which forms a part of a Base Station/Mobile Switching Center/Interworking function (BMI).
FIG. 3A shows that a 40 millisecond frame consists of six time slots. Slots 1–3 and 4–6 each comprise one TDMA Block. In the reverse direction from the mobile station to the base station, which is a case of most interest to this invention, the frames may be continuously transmitted. A given mobile station is assigned to transmit in one slot per frame for a half data rate case, and is assigned to transmit in two time slots for a full data rate case.
FIG. 1B illustrates one time slot of a Digital Control Channel (DCCH) that is transmitted to the BMI on a reverse channel. The slot starts with a bit guard (G) field, a six bit ramp (R) field, a 16-bit preamble (PREAM) field, and a 28-bit (14 symbol) SYNC word/time slot identifier field. The SYNC word/time slot identifier field is used for slot synchronization, equalizer training, and time slot identification. These fields are followed by two 122-bit DATA fields separated by a an additional synchronization field (i.e., the 24-bit SYNC+ field).
FIG. 1C illustrates one abbreviated time slot of the Digital Control Channel (DCCH) that is transmitted to the BMI on a reverse channel. It can be noted that the second data field is shortened to 78-bits, and a 44-bit abbreviated guard (AG) field is added during which time the mobile station maintains a carrier off condition.
IS-136.1, Rev. A, describes in Section 4.4.3.1 a channel encoding technique, in Section 4.4.3.2 an intraburst interleaving technique for the DATA fields, and in Section 4.4.3.2.2 an interleaving scheme and algorithm for transmitting the encoded data bits of the DATA fields in the normal length burst case of FIG. 1B, and the abbreviated length burst case of FIG. 1C.
Many modern digital cellular communications systems, including IS-136, support a Discontinuous-Transmission (DTX) mode of operation wherein the mobile station transmitter autonomously switches between two transmitter power levels while the mobile station is in a conversation state. By example, the mobile station can enter a DTX_Low power state during a pause in speech, thereby conserving battery power. In the IS-136 system DTX operation can be initiated by the mobile station when on either an analog voice channel or a digital traffic channel, if permitted by the base station in broadcast control channel signalling.
While in the DTX low power state the mobile station may still be required to periodically send channel quality measurements taken on adjacent base stations as a part of a Mobile Assisted Handoff (MAHO) operation. Reference in this regard can be had generally to IS-136.2, Rev. A, Section 2.4.5, and in particular to Section 2.4.5.3 “MAHO Operations with DTX Operation”.
Also while in the DTX low power transmitter state the BMI generates so-called “comfort noise” (CN) based on comfort noise parameters that are received from the mobile station. The use of comfort noise insures that the called party will hear a background noise component that is consistent with a normal conversation, as opposed to a complete and unnatural cessation of background noise due to the transmitting mobile station's transmitter being turned off or substantially reduced in power.
If the base station requires, the mobile station may not ramp down to the lowest possible transmitter power level, but may instead maintain a predefined minimum transmitter power such that adjacent base stations are able to use so-called digital locate receivers to monitor the mobile station's transmissions for power measurement and/or other purposes. In true DTX, the mobile station can completely terminate transmissions, thereby conserving a maximum amount of mobile station power.
However, the system specification defined in IS-136.1 and IS-136.2 does not include any definitive way for the mobile station to indicate to the BMI the beginning of the DTX mode. As a result, a decoder in the BMI cannot readily determine whether to employ bad frame masking because of lost slots due to fading (See IS-136.2, Section 2.2.2.2.3.2), or whether to generate comfort noise based on CN parameters. In addition, the BMI cannot locate the so-called hang-over slots, on which the BMI computes basic parameters for the background noise. The location of the hangover slots is important, since if the BMI does not receive the comfort noise parameters in the beginning of the slot, it must use the previously derived comfort noise until the next comfort noise parameters are received.
Due to the above-mentioned interleaving, the data bits of a last slot that is transmitted before entering the DTX_Low State have been unused. Currently in IS-136 a two slot interleaving technique is used (see IS-136.2, FIG. 2.1.3.3.3.2-1 and Section 2.1.3.3.4), which leaves 130 bits (half of a 260 bit slot) unused in the last slot. The last slot transmitted is typically a Fast Associated Control Channel (FACCH) slot, which is defined as a blank-and-burst channel used for signalling exchange between the mobile station and the base station.
It has been proposed in IS-136 (see contribution TR45.3.5/97.03.25.04) that a 68-bit truncated slot be used as a transmit (TX) slot when the mobile station is in the DTX_Low State. A revised contribution in this regard is TR45.3.6/97.06.10. Comfort Noise parameters have been proposed to be carried as FACCH messages on the FACCH channel.
In the existing Global System for Mobile Communication (GSM) DTX operation, the entry to the DTX mode is indicted by CN parameters sent with inband signaling. However, if for some reason this inband signalling is not received, the BMI will execute the bad frame masking procedure that repeats previous speech frames. In IS-136 the CN parameters are sent as FACCH messages, and similarly if the BMI fails to receive the CN parameters, or some other indication of the mobile station's entry to the DTX mode, the BMI will enter the bad frame masking procedure (see IS-136.2, Section 2.2.2.2.3.2).
As can be appreciated, the failure by the BMI to determine when the mobile station has entered the DTX mode can result in an objectionable audible signal being generated due to the repeating of the last received (good) speech frame.
A further existing problem of the DTX mode as currently specified relates to the number of slots that must be sent from the mobile station to the BMI while the mobile station is in the DTX_Low State. These slots are used, typically, for sending the CN parameters.